As disclosed in U.S. Pat. No. 2,297,691 to Carlson, an electrophotographic process employs a photoconductive material comprising a substrate which has been coated in a dark room with an insulating material which changes its electrical resistance depending on the amount of irradiation during imagewise exposure. Such a photoconductive material is generally given a uniform surface electrical charge after being adapted to darkness for a suitable period of time. The material is then exposed to a desired image by an irradiation pattern which has an effect of reducing surface electrical charge depending on relative energy contained in various portions of the irradiation pattern. The surface electrical charge or static latent image thus left behind on the surface of the photoconductive material layer (electrophotographic photosensitive layer) is then brought into contact with a suitable electroscopic displaying substrane or toner to develop a visible image.
Such a toner may be contained in either an insulating liquid or dry carrier. In either case, the toner may be attached to the surface of an electrophotographic photosensitive layer in accordance with an electrical charge pattern. The displaying substance thus attached may be fixed to the layer by a known means such as heat, pressure, and solvent vapor. The static latent image may be transferred to a second substrate (e.g., paper and film). Accordingly, the static latent image may be developed on such a second substrate.
Principle requirements in an electrophotographic process include that (1) the photoconductive material can be charged with a desired potential in a dark room, (2) the dissipation of electrical charge in a dark room is negligibly small, and (3) the electrical charge can be rapidly dissipated upon light irradiation.
Heretofore, photoconductive materials for electrophotographic photoreceptor that have been employed include selenium, cadium sulfide, and zinc oxide.
It is known that these inorganic materials have many advantages but, at the same time, have many disadvantages. For example, selenium, which is now widely used, satisfies the above requirements but is disadvantageous in that its complex production conditions entail high production costs. This material is also disadvantageous in that its poor flexibility makes it difficult to be worked into a belt-shaped form, and its high susceptibility to heat and mechanical impact requires careful handling. Cadmium sulfide or zinc oxide is dispersed in a binder such as a resin to be used as an electrophotographic photoreceptor. However, such an electrophotographic photoreceptor is disadvantageous in mechanical properties such as smoothness, rigidity, tensile strength, and abrasion resistance, and thus can not sufficiently repeatedly be used in its heretofore known embodiments.
In recent years, electrophotographic photoreceptors employing various organic materials have been proposed and put into practical use to eliminate these problems of inorganic materials. These electrophotographic photoreceptors include an electrophotographic photoreceptor made of poly-N-vinylcarbazole and 2,4,7-trinitrofluorene (see U.S. Pat. No. 3,484,237), an electrophotographic photoreceptor which comprises poly-N-vinyl carbazole sensitized with a pyrylium salt dye (see Japanese Patent Publication No. 25658/73), an electrophotographic photoreceptor mainly comprising an organic pigment (see Japanese Patent Application (OPI) No. 37543/72 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")), and an electrophotographic photoreceptor mainly comprising an eutectic complex made of a dye and a resin (see Japanese Patent Application (OPI) No. 10735/72).
If a proper binder is selected, an electrophotographic photoreceptor employing such an organic material can be applied to a substrate by a coating method. Therefore, such an electrophotographic photoreceptor provides an extremely high productivity, providing an inexpensive photoreceptor. Such an electrophotographic photoreceptor has improved mechanical properties and flexibility. Furthermore, when a dye and an organic pigment are properly selected, the photosensitive wavelength can freely be controlled. However, these electrophotographic photoreceptors cannot fully meet requirements for electrophotographic photoreceptor since they are low in photosensitivity and are not suitable for repeated use.